SANAA, Yemen — He has dispatched his own brother to death, hiding a bomb on him before he crossed into Saudi Arabia to target the kingdom’s chief counterterrorism official. He has tried to attack the United States three times in the past three years, building small, sophisticated and hard-to-detect devices in his workshop in the rugged terrain of southern Yemen.

Ibrahim Hassan al-Asiri, who hails from a middle-class Saudi family, is the top bombmaker for al-Qaeda’s Yemen branch. Only 30 years old, he represents the CIA’s worst fears: a highly skilled terrorist determined to attack the United States.

American officials believe Asiri’s latest bomb was designed to be smuggled onto a U.S.-bound aircraft last month. The non­metallic device had an advanced detonator and was superior to anything created by terrorists so far.

“Asiri is an evil genius,” said Rep. Peter T. King (R-N.Y.), chairman of the House Homeland Security Committee. “He is constantly expanding, he is constantly adjusting.”

U.S. intelligence and law enforcement officials have said Asiri built the bombs in the attempts to bring down a commercial flight near Detroit and two cargo planes headed for the United States. King said the latest bomb, part of a foiled plot, appeared to be “a modification of the previous ones.”

With the death of Osama bin Laden, Asiri is a key reason that U.S. officials consider Yemen’s al-Qaeda in the Arabian Peninsula, or AQAP, to be one of the most significant threats to the American homeland.

“He is highly determined and fully committed to attack America,” said Mustafa Alani, director of security and defense studies at the Gulf Research Center in Jiddah, Saudi Arabia. “For al-Qaeda, an attack inside the U.S. is worth 11 attacks outside. It has become their obsession.”

Over the past year, AQAP has taken advantage of Yemen’s political turmoil and seized large swaths of territory in the south. Yemen’s government and the Obama administration have responded with aerial assaults and drone strikes, targeting Asiri and other top operatives of AQAP.

Asiri built the “underwear” bomb worn by Umar Farouk Abdulmutallab when he tried to blow up a Northwest Airlines flight headed to Detroit on Christmas Day in 2009. A year later, Asiri built the printer­-cartridge bombs that al-Qaeda placed on Fedex and UPS planes destined for Chicago.

If it were not for a technical problem (Abdulmutallab’s device failed to detonate) or solid intelligence tips (Saudi counterterrorism officials alerted authorities in Dubai and Britain to intercept the cargo planes), Asiri would have succeeded in staging a catastrophic disaster in American skies.

“Ibrahim al-Asiri is the one who enabled the operations of al-Qaeda in Yemen to move from local attacks to international ones,” said Said Obaid, the Yemeni author of a book about AQAP.

A chemistry student

Asiri’s family hails from southwest Saudi Arabia, near the Yemeni border, the region that was home to several Sept. 11 hijackers. But nothing in Asiri’s childhood suggested that he and his brother Abdullah would turn to jihad. Born in Riyadh in 1982, Asiri is the son of a Saudi military officer.

“They used to listen to music and had a wide variety of friends, friends not like the ones they had later, when they became more religious,” his mother told the Saudi newspaper al-Watan in a 2009 interview.

A sister told al-Watan that her brothers became more devout and reclusive after the death of their elder brother in a car accident in 2000. “It was after that that they started swapping videotapes and cassettes on the mujaheddin,” she said.

Asiri studied chemistry in Riyadh, but when the United States invaded Iraq in 2003, he became more radicalized, like thousands in the Arab world. He tried to join al-Qaeda in Iraq to fight the U.S. occupation but was arrested by Saudi authorities as he tried to cross the border. He was imprisoned for nine months and released.

Soon after, according to Saudi officials, he and Abdullah launched a militant cell linked to the al-Qaeda wing in Saudi Arabia to plot attacks against oil pipelines, the royal family and security services. He was placed on the kingdom’s most-wanted list of al-Qaeda terrorists. By 2007, when al-Qaeda’s Yemen and Saudi branches merged to create AQAP, Asiri and his brother had crossed into Yemen.

There, Asiri learned to make bombs. Alani and other analysts say he was probably taught by a Pakistani bombmaker linked to al-Qaeda. But other analysts say Asiri appears to have taught himself, through manuals and other resources on the Internet.

“He seems to be largely ­self-educated, said Bruce Riedel, a former CIA officer who is a senior fellow at the Brookings Institution. “He has been able to miniaturize bombs so they are capable of being smuggled.”

Experimented with brother

Asiri’s brother became his first experiment. He had created a bomb unlike any other, using 100 grams of PETN, a white, powdery explosive that was virtually undetectable. Their target was Prince Mohammed bin Nayef, Saudi Arabia’s counterterrorism chief. In a video posted later on jihadist Web sites, Asiri embraced and kissed his brother and said goodbye.

Abdullah al-Asiri crossed the border and gained entry to Nayef’s palace by claiming he was defecting. But the explosion that killed Asiri only slightly wounded Nayef. Nonetheless, AQAP was emboldened. The group had never come so close to killing a member of the royal family.

With his brother dead, Asiri’s credentials in AQAP soared, Obaid said. Soon he was plotting his next attack, with Abdulmutallab, a Nigerian student he was introduced to by Anwar al-Awlaki, the U.S.-born cleric who was killed in an American drone strike last year. Asiri was initially thought to have died in the same strike.

A Justice Department sentencing memo from February in the Abdulmutallab case describes Asiri as having the Nigerian “practice the manner in which the bomb would be detonated; that is, by pushing the plunger of a syringe, causing two chemicals to mix, and initiating a fire (which would then detonate the explosive).”

In 2010, Asiri created even more sophisticated bombs, this time inside printer cartridges. They passed undetected through airport security. It took authorities in Dubai and Britain nine inspections before they found the devices. In March 2011, the Obama administration designated Asiri as a wanted terrorist.

Riedel said people he spoke to who examined the Detroit and cargo plane bombs said they show “learning and adaptation.” The most recent attempt, using no metal, was a further refinement to evade airport security scanners, Obaid said. “Al-Qaeda consider him a treasure that cannot be lost,” he said.

For that reason, he and other analysts say, Asiri is training the next generation of bombmakers in the event he is killed.

“The unfortunate reality is that Asiri is not the only one to worry about in AQAP,” said a U.S. official who spoke on the condition of anonymity to discuss the ongoing investigation. “He appears to be training others so that, if he is taken off the battlefield, his expertise won’t be lost.”

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